Biogas Installation for Production of Biogas from Biomass, and Methods for Operation of the Biogas Installation

ABSTRACT

A biogas installation for production of biogas from biomass and methods for starting and shutting down a fermenter are disclosed. When biomass in a fermenter is used up, biogas production must stop, the fermented biomass removed, and the fermenter filled with fresh biomass. Biogas production and utilisation is maintained for as long as possible. When the methane concentration in the biogas outlet falls below an upper limit, the biogas line is disconnected. The biogas/off-gas mixture is fed out via an exhaust chimney until the methane concentration has fallen to a lower limit. The fermenter to be shut down is purged with fresh air, and the off-gas/biogas/fresh air mixture is fed out via the exhaust chimney until the carbon-dioxide concentration in the off-gas/biogas/fresh air mixture has fallen to a first limit. The fermenter is then opened to unload the consumed biomass and load fresh biomass.

FIELD OF THE INVENTION

The invention relates to a biogas installation for production of biogasfrom biomass having at least one fermenter, to a method for shuttingdown a fermenter, and to a method for starting a fermenter

BACKGROUND OF THE INVENTION

So-called “dry fermentation” allows pourable biomasses from agriculture,from biological waste and from communal cultivated areas to be convertedto methane without having to convert the materials to a liquid substratewhich can be pumped. Biomasses with a dry substance component of up to50% can be fermented. This dry fermentation method is described, forexample, in EP 0 934 998.

In the case of “dry” fermentation, the material to be fermented is notstirred into a liquid phase as is the case, for example, with liquidfermentation of bio waste. Instead of this, the fermentation substratewhich has been introduced into the fermenter is kept moist all the timeby taking the percolate at the bottom of the fermenter away and sprayingit over the biomass again. This results in optimum living conditions forthe bacteria. During the recirculation of the percolate, the temperaturecan also be regulated, and it is possible to add additives for processoptimisation.

WO 02/06439 discloses a bioreactor or a fermenter in the form of aprefabricated garage which is operated using the principle of dryfermentation in the so-called batch process. In this case, after seedingwith already fermented material, the fermenter is filled with thefermentation substrate by means of tractor shovels. The fermentationcontainer is constructed in the form of a garage and is closed by agastight door. The biomass is fermented with air being excluded, with nofurther thorough mixing during the process, and with no additionalmaterial being supplied. The percolate which seeps out of the materialbeing fermented is drawn off via a drainage groove, is temporarilystored in a tank, and is sprayed over the fermentation substrate again,in order to moisturise it. The fermentation process takes place in themesophilic temperature range between 34 and 37° C., with the temperaturebeing created by means of bottom heating and wall heating.

The resultant biogas can be used to obtain electricity and heat in acogeneration system that generates heat and electric power at the sametime. In order to ensure that sufficient biogas is always available forthe cogeneration system, a plurality of fermentation containers areoperated with offset timings in the dry fermentation installation. Atthe end of the dwell time, the fermenter area is completely emptied andis then refilled. The fermented substrate is supplied to apost-composting process, resulting in an organic fertiliser that iscomparable to conventional compost.

The batch operation means the individual fermenters must be shut downfrom time to time, that is to say the biogas production must be stopped,the fermented biomass must be removed from the respective fermenter, andthe fermenter must be filled with fresh biomass, with biogas productionbeing resumed. During this process, it is necessary for safety reasonsto prevent an explosive biogas/air mixture from being created while theindividual fermenters are being loaded and unloaded.

For this purpose, it is known from EP 1301583 B for a fermenter that isin use to be flooded with off-gas containing carbon dioxide from thecogeneration system that is being operated with biogas, in the event ofan explosion risk, that is to say if air has entered the fermenter.

SUMMARY OF THE INVENTION

Starting out from a biogas installation according to EP 1301583 B, theobject of the present invention is to make safer the process ofunloading consumed biomass from the fermenter, and loading it with freshbiomass.

This object is achieved by the features of Claims 1, 10 and 14,respectively.

The biogas installation according to Claim 1 comprises the necessarycomponents to allow a fermenter to be shut down and unloaded safely, andlikewise to be started safely.

The measures according to Claim 10 result in the biogas production andutilisation being maintained for as long as possible even during theshutdown and purging with off-gas containing carbon dioxide, that is tosay the biogas/off-gas mixture of the fermenter to be shut down is stillsupplied to the biogas consumer until the quality of the mixture fallsbelow a predetermined level. Only when the methane concentration in thebiogas outlet falls below an upper limit value is the biogas lineleading to the biogas consumer disconnected from the biogas outlet. Thebiogas/off-gas mixture, which now just contains a small amount ofmethane, is fed out via an exhaust chimney. This is done until themethane concentration has fallen to a lower limit value, where thebiogas/off-gas mixture now contains virtually no methane. After this,the fermenter to be shut down is purged with fresh air instead of withoff-gas containing carbon dioxide, and the off-gas/biogas/fresh airmixture is fed out via the exhaust chimney until the carbon-dioxideconcentration in the off-gas/biogas/fresh air mixture has fallen to afirst limit value. Only then is the fermenter opened in order to unloadthe consumed biomass and in order to load the fermenter with freshbiomass again. The previous purging processes with off-gas and fresh airallow the fermenter to be opened, unloaded and loaded, without any riskto the operator.

According to one preferred refinement of the invention in Claim 11, whenthe methane concentration reaches the upper limit value, thebiogas/off-gas mixture is not emitted to the environment via the exhaustchimney but is passed to an off-gas flare where it is burnt off. Ifrequired, the off-gas flare can be supplied with additional fuel so thatcombustion always takes place. The biogas/off-gas mixture is burnt offuntil the methane concentration in the biogas/off-gas mixture fallsbelow a medium limit value, which is between the upper and the lowerlimit value.

According to one preferred refinement of the invention in Claim 12,while the biofermenter that has been shut down is being unloaded andloaded through the open loading and unloading opening, fresh air issucked in and the gas mixture that is sucked in is passed to the exhaustchimney via the purging gas inlet or via the biogas outlet.Alternatively, a specific fresh-air extraction connection can also beprovided in the fermenter.

The advantageous refinement of the invention in Claim 13 ensures thatfresh air is sucked in continuously while the fermenter is beingunloaded and loaded.

The method according to the invention for (re)starting the fermenterwhich has been shut down in Claims 14 and 15 safely prevents anexplosive biogas/air mixture being formed during starting.

This fermenter which has been started again is connected to the biogasline at a fourth methane concentration limit value, which is the same asthe upper limit value, Claim 16.

The off-gas for purging the fermenter to be shut down is provided, forexample, by an internal combustion engine, Claim 17.

According to one preferred embodiment of the invention in Claim 18, theoff-gas which contains carbon dioxide is provided by a biogas processingdevice connected downstream from the at least one fermenter.

The other dependent Claims relate to advantageous refinements of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, features and advantages of the invention will becomeevident from the following description of exemplary embodiments withreference to the drawings, in which:

FIGS. 1 to 7 show schematic illustrations of various operating statesduring the shut down process and during (re)starting of a fermenter;

FIG. 8 shows a schematic illustration of a second embodiment of theinvention with a fermenter; and

FIGS. 9 to 15 show schematic illustrations of various operating statesof a biogas installation having three fermenters during the shut downprocess and while a fermenter is being (re)started.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 7 show a first embodiment of a biogas installation accordingto the present invention with a single fermenter 2. The fermenter 2 hasa cuboid shape and is constructed approximately in the form of aprefabricated garage. The fermenter 2 can be filled with biomass 6, andcan be emptied again by means of a tractor shovel through a loading andunloading opening 4 which extends over one of the end faces of thecuboid fermenter 2. Reference is made to WO 02/06439 with regard todetails of the construction of the fermenter 2.

The fermenter 2 also has a biogas outlet 8, which can be connected viavalves 10 to a biogas line 12, a first biogas/off-gas line 14 and asecond biogas/off-gas line 16. The biogas line 12 leads to acogeneration system 18 as a biogas utilisation device. The firstbiogas/off-gas line 14 leads to a bio off-gas chimney 20. The secondbiogas/off-gas line 16 leads to an off-gas flare 22. Furthermore, thefermenter 2 has a purging gas inlet 24, which can be connected viavalves 10 to an off-gas line 26 or to a fresh air line 28. An off-gasfan 27 is arranged in the off-gas line 26, and can be used to pumpoff-gas into the fermenter 2. A fresh air fan 29 is arranged in thefresh air line 28 in order to suck fresh air in from the environment.Off-gas containing carbon dioxide is passed via the off-gas line 26 aspurging gas into the fermenter 2, and fresh air is passed into thefermenter 2 via the fresh air line 28.

The valves 10 are connected to a control device 30, and are opened orclosed by the control device 30. The control device 30 is also connectedto a first measurement sensor 32, which is arranged in the biogas outlet8 and detects the methane concentration in the respective gas mixture.The control device 30 is also connected to a second measurement sensor34, which is likewise arranged in the biogas outlet 8 and detects thecarbon-dioxide concentration in the respective gas mixture. The controldevice 30 is also connected to a third measurement sensor 36, which isarranged in the biogas outlet 8 and detects the gas volume flow in thebiogas outlet. If required, the extraction of gas from the fermenter 2can be assisted by a fan 38 which is arranged in the biogas outlet.

FIGS. 1 to 7 show various phases of the shut down process and process ofstarting the fermenter 2, with active lines and positions of componentsbeing illustrated by solid lines, while lines and positions ofcomponents which are inactive or are shut down are illustrated by dashedlines.

FIG. 1 shows the first phase of shutting down the fermenter 2, in whichoff-gas containing carbon dioxide is pumped via the off-gas line 26 andthe purging gas inlet 24 into the interior of the fermenter 2. Thebiogas outlet 8 is connected, as before, to the biogas line 12, so thatthe biogas/off-gas mixture is passed on to the cogeneration system 18.

Only when the methane concentration detected by the first measurementsensor 32 in the biogas outlet 8 has fallen below an upper limit valueis the valve 10 in the biogas line 12 closed by the control device 30,and the valve 10 in the second biogas/off-gas line 16 is opened, as isillustrated in FIG. 2. In this second phase of shutting down thefermenter 2, the biogas/off-gas mixture is burnt in the off-gas flare22. If required, this combustion process can be assisted by addingadditional fuel.

When the methane concentration detected by the first measurement sensor32 in the biogas outlet 8 has fallen below a medium limit value, thevalve 10 in the second biogas/off-gas line 16 is closed by the controldevice 30 and the valve 10 in the first biogas/off-gas line 14 isopened, as is illustrated in FIG. 3. In this third phase of shuttingdown the fermenter 2, the biogas/off-gas mixture is emitted to theenvironment via the off-gas chimney 20.

When the methane concentration detected by the first measurement sensor32 in the biogas outlet 8 has fallen below a lower limit value, thevalve 10 in the off-gas line 26 is closed by the control device 30 andthe valve 10 in the fresh air line 28 is opened, as is illustrated inFIG. 4. In this fourth phase of shutting down the fermenter 2, fresh airis pumped into the fermenter 2 via the fresh air line 28 and the purginggas inlet 24. The off-gas/air mixture is emitted further to theenvironment via the biogas outlet 8 and the first biogas/off-gas line 14in the off-gas chimney 20.

When the carbon-dioxide concentration detected by the second measurementsensor 34 in the biogas outlet 8 has fallen below a first limit value,the valve 10 in the fresh air line 28 is closed by the control device30, and the loading and unloading opening 4 is opened, as is illustratedin FIG. 5. At the same time, the fan 38 is used to suck fresh air in viathe open loading and unloading opening, and to emit it to theenvironment via the off-gas chimney 20. This prevents biogas residueswhich the fermented biomass still contains from representing a risk tothe operator during the unloading process.

Once the fermenter 2 has been loaded with fresh biomass again, theloading and unloading opening 4 is closed, the connection between thebiogas outlet 8 and the off-gas chimney 20 is maintained via the firstbiogas/off-gas line 14, and the control device 30 opens the valve 10 inthe off-gas line 26, so that the off-gas which contains carbon dioxideis pumped into the fermenter 2, see FIG. 6. This is continued until thecarbon-dioxide concentration detected by the second measurement sensor34 in the biogas outlet 8 reaches or exceeds a second limit value.

When the carbon-dioxide concentration has reached this second limitvalue, the control device 30 closes the valve 10 in the off-gas line 26and in the first biogas/off-gas line 14, and opens the valve 10 in thebiogas line 12, as is illustrated in FIG. 7. The biogas production phasehas therefore been reached again, and the biogas produced in thefermenter 2 is supplied via the biogas line 12 to the cogenerationsystem 18.

In the embodiment described above, all the measurement sensors 32, 34,36 are arranged in the biogas outlet 8. Alternatively, the second andthird measurement sensors 24, 36 can also be arranged in the first andsecond biogas/off-gas lines 14, 16, respectively. FIG. 8 shows analternative refinement of the invention, which differs from theembodiment shown in FIGS. 1 to 7, in that the first and secondbiogas/off-gas lines 14, 16 are combined to form a common biogas/off-gasline 40, before they open into the biogas outlet 8. The secondmeasurement sensor for detection of the carbon-dioxide concentration isarranged in the common biogas/off-gas line 40, and the third measurementsensor 36 is arranged in the first biogas/off-gas line 14. Apart fromthis, the second embodiment of the invention corresponds to the firstembodiment. The method of operation is also identical.

FIGS. 9 to 15 show a third embodiment of a biogas installation accordingto the present invention, in which three fermenters 2-1, 2-2 and 2-3 areprovided, and are operated in parallel. Mutually correspondingcomponents are provided with the same reference symbols. In the biogasinstallation shown in FIGS. 9 to 15, each of the three fermenters 2-i isprovided with a purging gas inlet 24-1, 24-2 and 24-3, which can each beshut off by a valve 10. The three purging gas inlets 24-i are combinedto form a common purging gas inlet 42. An off-gas line 26 and a freshair line 28 open into the common purging gas inlet 42, and can each beshut off by a valve 10.

Each of the three fermenters 2-i is provided with a biogas outlet 8-1,8-2 and 8-3, which can each be shut off by a valve 10. The firstbiogas/off-gas line 14 to the off-gas chimney 20 and the secondbiogas/off-gas line 16 to the off-gas flare 22 are combined to form acommon biogas/off-gas line 40 in which a fan 38 is arranged. Downstreamfrom the fan 38, the common biogas/off-gas line 40 splits into a first,a second and a third biogas/off-gas line element 40-1, 40-2 and 40-3.The first biogas/off-gas line element 40-1 opens between the valve 10and the first biofermenter 2-1 into the first biogas outlet 8-1. Thesecond biogas/off-gas line element 40-2 opens between the valve 10 andthe second biofermenter 2-2 into the second biogas outlet 8-2. The thirdbiogas/off-gas line element 40-3 opens between the valve 10 and thethird biofermenter 2-3 into the third biogas outlet 8-3. The threebiogas/off-gas line elements 40-1, 40-2 and 40-3 can each be shut off bya valve 10. The three biogas outlets 8-1, 8-2 and 8-3 open into a commonbiogas line 12, which leads to a cogeneration system or combined heatand power unit 18. An exhaust line 44 from the cogeneration system 18opens into a second off-gas chimney 46. The off-gas line 26 is connectedvia a 3-way valve 48 to the exhaust line 44, that is to say the off-gascontaining carbon dioxide which occurs in the cogeneration system 18 isused to purge a fermenter 2-i that is to be shut down. The 3-way valveallows the volume flow of the off-gas which is sent via the off-gas line26 in order to purge a fermenter 2-i, and the amount of off-gas which isemitted via the second off-gas chimney 46 to the environment, to beregulated.

A first measurement sensor 32 is arranged in the common biogas line 12in order to detect the methane concentration. A second measurementsensor 34 for detection of the carbon-dioxide concentration, a thirdmeasurement sensor 36 for detection of the volume flow, and a fourthmeasurement sensor 50 for detection of the methane concentration, arearranged in the common biogas/off-gas line 40, downstream from the fan38 in the flow direction. The four measurement sensors 32, 34, 36 and 50are connected to a control device 30. The various valves 10 are likewiseconnected to the control device. These control lines are not shown inFIGS. 9 to 15, for clarity reasons.

FIGS. 9 to 15 illustrate the processes for shutting down and restartingthe second fermenter 2-2, with FIGS. 9 to 15 showing the same phases andoperating states as FIGS. 1 to 7. The biogas production in the first andthird fermenters 2-1 and 2-3 takes place continuously while the secondfermenter 2-2 is being shut down and started again.

FIG. 9 shows the first phase of shutting down the fermenter 2-2, inwhich off-gas containing carbon dioxide from the cogeneration system 18is pumped via the 3-way valve 48 and the off-gas line 26, the off-gasfan 27 and the second purging gas inlet 24-2 into the interior of thefermenter 2-2. The second biogas outlet 8-2 is connected, as before, tothe common biogas line 12, so that the biogas/off-gas mixture issupplied further to the cogeneration system 18.

Only when the methane concentration detected by the first measurementsensor 32 in the common biogas line 12 has fallen below an upper limitvalue is the valve 10 in the second biogas outlet 8-2 closed by thecontrol device 30, and the valve 10 in the second biogas/off-gas lineelement 40-2 and in the second biogas/off-gas line 16 is opened, as isillustrated in FIG. 10. In this second phase of shutting down thefermenter 2-2, the biogas/off-gas mixture is burnt in the off-gas flare22. If necessary, this combustion process can be assisted by addingadditional fuel.

When the methane concentration detected by the fourth measurement sensor50 in the common biogas/off-gas line 40 falls below a medium limitvalue, the valve 10 in the second biogas/off-gas line 16 is closed bythe control device 30 and the valve 10 in the first biogas/off-gas line14 is opened, as is illustrated in FIG. 11. In this third phase ofshutting down the fermenter 2-2, the biogas/off-gas mixture is emittedto the environment via the off-gas chimney 20.

When the methane concentration detected by the fourth measurement sensor50 in the common biogas/off-gas line 40 has fallen below a lower limitvalue, the valve 10 in the off-gas line 26 is closed by the controldevice 30, the 3-way valve 48 is switched appropriately, and the valve10 in the fresh air line 28 is opened, as is illustrated in FIG. 12. Inthis fourth phase of shutting down the fermenter 2-2, the fresh air fan29 pumps fresh air into the fermenter 2-2 via the fresh air line 28 andthe purging gas inlet 24. The off-gas/air mixture is furthermore emittedto the environment via the second biogas outlet 8-2, the secondbiogas/off-gas line element 40-2, the common biogas/off-gas line 40 andthe first biogas/off-gas line 14, in the off-gas chimney 20. Ifnecessary, this can be assisted by the fan 38.

When the carbon-dioxide concentration detected by the second measurementsensor 34 in the common biogas line 40 has fallen below a first limitvalue, the valve 10 in the fresh air line 28 is closed by the controldevice 30, and the fresh air fan 29 is switched off, as is illustratedin FIG. 13. The loading and unloading opening, which is not illustratedin FIGS. 9 to 15, is opened. At the same time, fresh air is sucked in tothe common biogas/off-gas line 40 via the fan 38 and via the openloading and unloading opening, and is emitted to the environment via theoff-gas chimney 20. This prevents any biogas residues which thefermented biomass still contains representing a risk to the operatorduring the unloading process. Exhaust gases from a tractor shovel whichis used for loading and unloading are therefore also sucked out.

Once the fermenter 2-2 has been loaded with fresh biomass again, theloading and unloading opening is closed, the connection between thesecond biogas outlet 8-2 and the off-gas chimney 20 via the secondbiogas/off-gas line element 40-2, the common biogas/off-gas line 40 andthe first biogas/off-gas line 14 is maintained, and the control device30 opens the valve 10 in the off-gas line 26, and switches the 3-wayvalve 48 in the exhaust line 44 of the cogeneration system 18, so thatoff-gas containing carbon dioxide is pumped into the fermenter 2-2, seeFIG. 14. This process continues until the carbon-dioxide concentrationdetected by the second measurement sensor 34 in the commonbiogas/off-gas line 40 reaches or exceeds a second limit value.

When this second limit value for the carbon-dioxide concentration hasbeen reached, the control device 30 closes the valve 10 in the off-gasline 26, switches the 3-way valve 38, closes the valve 10 in the secondbiogas/off-gas line element 40-2 and opens the valve 10 in the secondbiogas outlet 8-2, as is illustrated in FIG. 15. This means that thesecond fermenter 2-2 has also once again reached the phase of biogasproduction, and the biogas produced in the fermenter 2-2 is supplied viathe biogas line 12 to the cogeneration system 18. The biogas outlet 8-2is not connected to the common biogas line 12 until the methaneconcentration detected by the fourth measurement sensor 50 has reached afourth limit value. This fourth limit value matches the upper limitvalue.

The valve 10 in the off-gas line 26 may be omitted since its functioncan also be carried out by the 3-way valve 48.

Instead of directly connecting the biogas line 12 to the cogenerationsystem 18 it may be connected first to a gas processing device (notshown) to improve the gas quality. The biogas with improved quality isthen fed to the congeneration system 18. Exhaust gas or off-gas from thegas processing device may be fed to the off-gas line 26.

The following text gives examples of numerical values of the variouslimit values:

Methane concentration:

-   -   upper limit value 30% to 50%    -   medium limit value 10% to 20%    -   lower limit value 0% to 3%    -   fourth limit value 30% to 50%        Carbon-dioxide concentration:    -   first limit value 0.5% to 2%    -   second limit value 5% to 15%

The off-gas volume flow in the off-gas line 26 is between 150 and 1000m³/h, depending on the size of the fermenter and the amount of off-gasavailable. The fresh air volume flow in the fresh air line 28 is between1000 and 5000 m³/h.

1. Biogas installation for production of biogas having at least onefermenter (2) which operates on the principle of dry fermentation forproduction of biogas in the batch mode with a biogas outlet (8) and apurging gas inlet (24); a biogas line (12) that can be connected to thebiogas outlet (8); an off-gas line (26) by means of which off-gascontaining carbon dioxide can be supplied to the purging gas inlet (24);an off-gas chimney (20) which can be connected to the biogas outlet (8)via a first biogas/off-gas line (14); an off-gas flare (22) which can beconnected via a second biogas/off-gas line (16) to the biogas outlet(8); a fresh air line (28) which can be connected to the purging gasinlet (24); a control device (30) for connection of the biogas outlet(8) to the biogas line (12) or to the bio off-gas chimney (20) via thefirst biogas/off-gas line (14) or to the off-gas flare (22) via thesecond biogas/off-gas line (16) and for connection of the purging gasinlet (24) to the off-gas line (26) or to the fresh air line (28); and ameasurement device (32, 34) which is connected to the control device(30) and has a first measurement sensor (32) for detection of themethane concentration and a second measurement sensor (34) for detectionof the carbon-dioxide concentration in the gas mixture emerging from theat least one fermenter (2).
 2. Biogas installation according to claim 1,characterised in that the measurement device (32, 34) is arranged in thebiogas outlet (8).
 3. Biogas installation according to claim 1,characterised in that a plurality of fermenters (2-i) are provided,whose biogas outlets (8-i) open into the common biogas line (12), and inthat the first measurement sensor for detection of the methaneconcentration is arranged in the common biogas line.
 4. Biogasinstallation according to claim 3, characterised in that the biogasoutlets (8-i) can be connected selectively via a common biogas/off-gasline (40) to the off-gas chimney (20) or to the off-gas flare (22), andin that the second measurement sensor for detection of thecarbon-dioxide concentration is arranged in the common biogas/off-gasline.
 5. Biogas installation according to claim 1, characterised in thatthe off-gas line (26) supplies exhaust gas from an internal combustionengine.
 6. Biogas installation according to claim 1, characterised inthat the biogas line (12) connects to a biogas utilisation device whichproduces off-gas containing carbon dioxide.
 7. Biogas installationaccording to claim 5, characterised in that the biogas utilisationdevice comprises a cogeneration system (18).
 8. Biogas installationaccording to claim 5, characterised in that the biogas utilisationdevice comprises a fuel cell.
 9. Biogas installation according to claim5, characterised in that the biogas utilisation device comprises a gasprocessing device (44).
 10. Method for shutting down a fermenter in abiogas installation according to one of the preceding claims, having thefollowing method steps: a) maintaining the connection between the biogasoutlet (8) and the biogas line (12); b) connecting the off-gas line (26)to the purging gas inlet (24) of the fermenter (2) to be shut down; c)purging of the fermenter (2) to be shut down with off-gas from theoff-gas line (26) until the methane concentration detected by the firstmeasurement sensor (32) has fallen to an upper limit value; d)disconnenting the biogas line (12) from the biogas outlet (8) of thefermenter (2) to be shut down; e) connecting the biogas outlet (8) ofthe fermenter (2) to be shut down to the first biogas/off-gas line (14),and supply of the off-gas/biogas mixture to the off-gas chimney (20)until the methane concentration detected by the first measurement sensor(32) has fallen to a lower limit value; f) disconnecting the off-gasline (26) from the purging gas inlet (24) of the fermenter (2) to beshut down; g) connecting the fresh air line (28) to the purging gasinlet (24) of the fermenter (2) to be shut down, and supply of fresh airinto the fermenter (2) to be shut down, until the carbon-dioxideconcentration detected by the second measurement sensor (34) has fallento a first limit value; and h) opening of the loading and unloadingopening (4) of the fermenter (2) that has been shut down.
 11. Methodaccording to claim 10, characterised in that the following method stepsare carried out between method step d) and method step e): d1)connecting the biogas outlet (8) of the fermenter (2) to be shut down tothe second biogas/off-gas line (16), and supply of the off-gas/biogasmixture to the off-gas flare (22) until the methane concentrationdetected by the first measurement sensor (32) has fallen to a mediumlimit value which is between the upper and the lower limit value; andd2) disconnecting the biogas outlet (8) of the fermenter (2) to be shutdown from the second biogas/off-gas line (16).
 12. Method according toclaim 10, further characterised by the following method steps: i)connecting the fresh air line (28) to the purging gas inlet (24) and/orto the biogas outlet (8); and j) supplying fresh air into the fermenter(2) that has been shut down via the loading and unloading opening (4) bysucking out via the purging gas inlet (24) and/or the biogas outlet (8)while the fermenter (2) that has been shut down is being unloaded andloaded.
 13. Method according to claim 12, characterised in that, duringmethod step j), the volume flow of the air/biogas/off-gas mixture suckedout via the purging gas inlet (24) and/or the biogas outlet (8) ismonitored by the control device (30).
 14. Method for starting afermenter (2) which has been freshly loaded with biomass according toclaim 1, having the following method steps: a) closing of a loading andunloading opening (4); b) connecting the biogas outlet (8) to the firstbiogas/off-gas line (14); c) connecting the off-gas line (26) to thepurging gas inlet (24) of the fermenter (2) to be started and supply ofoff-gas to the fermenter (2) to be started until the carbon-dioxideconcentration detected by the second measurement sensor (34) reaches asecond limit value; d) disconnecting the off-gas line (26) from thepurging gas inlet (24); e) disconnecting the first biogas/off-gas line(14) from the biogas outlet (8); f) connecting the biogas line (12) tothe biogas outlet (8).
 15. Method according to claim 14, characterisedin that method step f) is carried out when the methane concentrationdetected by the first or fourth measurement sensor (32; 50) exceeds afourth limit value.
 16. Method according to claim 15, characterised inthat the fourth limit value of the methane concentration is equal to theupper limit value of the methane concentration.
 17. Method according toclaim 10, characterised in that the off-gas line (26) is connected tothe exhaust pipe of an internal combustion engine.
 18. Method accordingto claim 10, characterised in that the off-gas line (26) is connected tothe exhaust pipe of a biogas processing device which produces an off-gascontaining carbon dioxide.
 19. Method according to claim 10,characterised in that the off-gas line (26) is connected to the exhaustof a fuel cell.